Article

Influence of initial heat treatment of 17-4 PH stainless steel on gas nitriding kinetics

Surface & Coatings Technology - SURF COAT TECH 05/2008; 202(19). DOI: 10.1016/j.surfcoat.2008.04.058

ABSTRACT Results of the investigation of nitrided layers on 17-4 PH type precipitation hardening stainless steel are presented in this paper. The layers have been produced in the process of gas nitriding in a partly dissociated ammonia at temperatures between 410 and 570 °C. Hydrogen chloride admixture to active atmosphere was used as a surface activator. Structure of the nitrided layers were examined using scanning and transmission electron microscopy, X-ray microanalysis (EDX and WDX), and X-ray diffraction. The influence of the initial steel heat treatment on the nitriding kinetics has been considered. 17-4 PH stainless steel was nitrided at various heat treatment conditions, i.e. after solution treatment or ageing at different temperatures. The influence of precipitation processes taking place during the heat treatment before nitriding on the diffusive process kinetics was proven. It was found that, that increasing of steel ageing temperature up to 600 °C before nitriding effects on an increasing of the nitriding kinetics.

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    ABSTRACT: The effects of process parameters on the microstructure, microhardness, and dry-sliding wear behavior of plasma nitrided 17-4PH stainless steel were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and wear testing. The results show that a wear-resistant nitrided layer was formed on the surface of direct current plasma nitrided 17-4PH martensitic stainless steel. The microstructure and thickness of the nitrided layer is dependent on the treatment temperature rather than process pressure. XRD indicated that a single α N phase was formed during nitriding at 623 K (350 °C). When the temperature increased, the α N phase disappeared and CrN transformed in the nitrided layer. The hardness measurement demonstrated that the hardness of the stainless substrate steel increased from 320 HV0.1 in the untreated condition increasing to about 1275HV0.1 after nitriding 623 K (350 °C)/600 pa/4 hours. The extremely high values of the microhardness achieved by the great misfit-induced stress fields associated with the plenty of dislocation group and stacking fault. Dry-sliding wear resistance was improved by DC plasma nitriding. The best wear-resistance performance of a nitrided sample was obtained after nitriding at 673 K (350 °C), when the single α N-phase was produced and there were no CrN precipitates in the nitrided layer.
    Metallurgical and Materials Transactions B 44(2). · 1.32 Impact Factor
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    ABSTRACT: 17-4PH stainless steel was plasma nitrocarburized at 460°C for improving its mechanical properties without compromising its desirable corrosion resistance. The plasma nitrocarburized layers were studied by optical microscope, X-ray diffractometer, microhardness tester, pin-on-disc tribometer and the anodic polarization method in a 3.5% NaCl solution. The experimental results show that the nitrocarburized layer depths increase with increasing duration time and the layers growth conform approximately to the parabolic law. The phases in the nitrocarburized layer are mainly of γ′-Fe4N and α′-Fe with traces of CrN phase. The surface hardness of the modified specimen is more than 1200HV, which is three times higher than that of untreated one. The friction coefficient and corrosion resistance of the specimen can be apparently improved by plasma nitrocarburizing. With the increase of duration time, the surface hardness slightly decreases whereas the friction coefficient and corrosion resistance of the modified specimen are first increase and then decrease. The 8h treated specimen has the lowest friction coefficient and the best corrosion resistance in the present test conditions.
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